Social rank and sex ratio of captive long‐tailed macaque females (Macaca fascicularis)

Abstract

Variation in birth sex ratios in primates can be accounted for by two hypotheses: the local resource competition hypothesis [Silk: American Naturalist 121:56-66, 1983] and the hypothesis of Trivers & Willard [Science 179:90-92, 1973] concerning the maternal effect on the quality of a male. We examined the effects of female dominance rank on aspects of reproduction in three well-established captive groups of long-tailed macaques (Macaca fascicularis). High-ranking females produced a higher proportion of sons than low-ranking females, and factors other than rank did not have significant effects on birth sex ratios. Interbirth intervals following daughters were longer than those following sons, but they were independent of the mother's rank. The sons of high-ranking mothers had better survival prospects than sons of low-ranking mothers in some of the groups; no such difference was found for daughters. Overall, there was no sex difference in survival up to 5 years of age. These results support the Trivers-Willard hypothesis rather than the local resource competition hypothesis. An analysis of interbirth intervals suggested that the deviation in birth sex ratio is already established at conception.